Explanation

Among the given options, oxygen is the most abundant element: it forms about 46.6 percent of the Earth's crust by mass and roughly 30 percent of the whole Earth, second only to iron, which is concentrated in the core and is not among the options. Silicon (about 27.7 percent of the crust) and aluminium (about 8.1 percent) follow, while carbon is far less abundant. Hence oxygen is the correct choice.